A Nd—Fe—B based sintered magnet (so-called neodymium magnet) is made of a combination of iron and elements of Nd and B that are inexpensive and abundant natural resources and stably obtainable, and can thus be manufactured at a low cost and additionally has high magnetic properties (its maximum energy product is about 10 times that of ferritic magnet). Accordingly, the Nd—Fe—B based sintered magnets have been used in various kinds of articles such as electronic devices and have recently come to be adopted in motors and electric generators for hybrid cars. The amount of their uses is on the increase.
The Nd—Fe—B based magnets are mainly manufactured in a powder metallurgy method. In this method, Nd, Fe, and B are first formulated in a predetermined composition ratio, melted, and cast to thereby manufacture an alloy raw material. Then, the alloy raw material is once coarsely ground by, e.g., hydrogen grinding step and successively fine-ground by, e.g., jet mill fine grinding step, thereby obtaining raw metal (in powder form, hence referred to as raw metal powder). Then, the obtained raw metal powder is oriented in the magnetic field (magnetic field orientation), and is compression-molded in a state in which the magnetic field is being applied, thereby obtaining a molded (or formed) body. This molded body is sintered under predetermined conditions to thereby manufacture a sintered magnet.
In the compression molding method in the magnetic field, a uniaxial pressurizing type of compression molding machine is generally used. This compression molding machine is so arranged that raw metal powder is filled into a cavity (filling chamber) formed in a through hole or through hole in the die, and is pressed (or urged) by a pair of upper and lower punches in a vertical direction to thereby mold the raw metal powder. At the time of compression molding by the pair of punches, a high or superior orientation cannot be obtained due to friction among the particles in the raw metal powder filled into the cavity, or due to friction between the raw metal powder and the wall surfaces of the metallic mold set in position in the punch. There was thus a problem in that improvement in the magnetic properties cannot be attained.
As a solution to the above problem, there is known a compression molding method in which, after having filled the cavity with raw metal powder, at least one of the upper punch and the lower punch is subjected to vibrations in the direction of pressurizing (pressing direction) at the time of orientation in the magnetic field. In this compression molding method, by charging the magnetic field to the raw metal powder while vibrating the raw metal powder by the upper punch or the lower punch, the friction among the particles of the raw metal powder filled into the cavity can be changed from the static friction to the dynamic friction. The friction among the particles of the raw metal powder is thus reduced to thereby improve the flowability of the raw metal powder. As a result, the raw metal powder can be moved so as to be more aligned in the direction of orientation in the magnetic field, whereby the orientation can be improved (see patent document 1).    Patent document 1: International Patent Publication No. 2002/60677 (see, e.g., what is described in claims)